1. Field of the Invention
The present invention relates to a signal processing method which is suitable when it is used, particularly, in the case where data for a computer is recorded to an optical disc or the case where data recorded on an optical disc is reproduced.
2. Description of the Prior Art
As a memory medium for recording a digital video signal which was compressed by an MPEG (Moving Picture Experts Group) or the like, a DVD (digital video disc) has been developed. The DVD is an optical disc which has the same diameter as that of a CD (compact disc) and in which a recording density has been further improved by a progress in realization of a short wavelength of a laser beam, an increase in numerical aperture NA of an objective lens, and an improvement of processes for a digital modulation and an error correction encoding. Even in the case of a single-layer disc, the DVD has an extremely large data memory capacity for example about 3.7 Gbytes. As a form of the DVD, there has been proposed a disc which is recordable and reproducible by using an MO disc or a phase-change type optical disc in addition to a read only optical disc.
It has been proposed that such a DVD be used as an external memory device of a computer. That is, as an external memory device of the computer, attention is paid to an optical disc drive having advantages such as large capacity and high-speed access. A CD-ROM and a CD-I (CD-Interative) constructed in a manner such that data is recorded to a CD, MD (mini disc) data which is recorded and reproduced to/from an MD (mini disc), and the like have already been proposed. Since the DVD has an extremely large data memory capacity of about 3.7 Gbytes, the DVD is expected to be an external memory medium which can store data of a larger capacity.
In the DVD, in order to protect recording data, an error correction encoding process is performed. Since sequential data such as video data is handled in the DVD, a cross interleave code of a convolution type is used. However, when the convolution code is used, it is difficult to access the data on a sector unit basis and to read and write the data. In the case where the DVD is used as an external memory medium for recording and reproducing data of the computer, it is necessary to access the data on a sector unit basis and to read and write the data, so that it is necessary to use a code of a block completion type.
Namely, FIG. 9 shows an example of processes in the case of generating the convolution type cross interleave code. As shown in FIG. 9, a parity P of, for example, eight bytes is added to, for example, 162 bytes in the vertical direction in a C1 sequence. In a C2 sequence, a parity Q of, for example, 14 bytes is added in the oblique direction. By adding data of 148 bytes in the vertical direction, parity P of 8 bytes, parity Q of 14 bytes, the number of data in the vertical direction is equal to 170 bytes. The data recorded on the disc is read out therefrom synchronously with the frame. One frame is equal to 85 bytes. The data of 170 bytes in the vertical direction is equal to an amount of just two frames.
As mentioned above, although the convolution type cross interleave code is used in the DVD, it is difficult to access every block of the data and read and write the data by using the convolution type code. It is, therefore, considered to use a cross interleave code of the block completion type.
FIG. 10 shows an example of processes in the case of generating the block completion type cross interleave code. In order to realize a common construction with the case of the above-mentioned convolution code, the number of data in the vertical direction is set to 148 bytes, the parity P is set to eight bytes, the parity Q is set to 14 bytes, and the total number of data in the vertical direction is set to 170 bytes. Since one sector is set to, for example, 16 kbytes, the number of data in the horizontal direction is set to 112 bytes (148.times.112=16576 bytes). When the block completion type cross interleave code is used, a length of interleave is longer than a width of block. Therefore, as shown in FIG. 10, when the number of data in the horizontal direction reaches the length of 112 bytes, an aliasing of the C2 sequence occurs. As shown in FIG. 10, when the block completion type cross interleave code is used, data can be read and written at every block such that it is suitable for a case where the DVD is used as an external memory medium for recording and reproducing data for the computer.
As a sector size in the case where data is recorded and reproduced to/from a data recording medium such as a DVD or the like becomes large, the disc can meet requirements for a high density and high reliability. In association with it, the file size to be handled becomes more and more large. Therefore, in recent years, there is a tendency to increase the sector size of data recording medium such that a size of one sector has been changed from initial 512 bytes to 1024 bytes and, further, to 2 kbytes.
However, since the access to the recording medium is performed on a sector unit basis, as the sector size becomes large, it takes a long time for reading and reproducing. There is, consequently, a problem such that an accessing speed including error correction encoding and decoding processes decreases. In consideration of problem of a compatibility with a sector of an existing data recording medium, there is also a demand to enable data to be handled by a small sector.
In the case of recording data to the DVD, however, when the number of sectors is reduced, the reliability of the data deteriorates.
That is, in the above-mentioned example, the sector size is set to 16 kbytes. When the sector size is set to 16 kbytes, as shown in FIG. 10, an aliasing of the C2 sequence occurs once.
Since the block shown in FIG. 10 has 16 kbytes, when such a block is divided into eight portions, a sector of 2 kbytes can be constructed. However, as shown in FIG. 11, when the block of 16 kbytes is divided into eight portion, the sector of 2 kbytes (14.times.148=2072 bytes) is constructed and the error correction encoding is performed by using the cross interleave code in a manner similar to the foregoing example, many aliasings occur in the C2 sequence. Thus, a sufficient error correction cannot be performed.
As mentioned above, when the sector size is increased, although the reliability of data is improved, there is also a drawback such that the accessing speed becomes slow.
On the contrary, when the sector size is reduced, although the accessing speed is raised, there is a drawback such that the reliability of data deteriorates.